Variable light beam scanning apparatus

ABSTRACT

A light beam scanning apparatus in which light beams are modulated by image information signals and the modulated light beams are directed onto a photosensitive body consists of a light source which produces light beams, a first scanning unit which causes the light beams produced by the light source to scan in a first direction with respect to a photosensitive body, a second scanning unit which causes the light beams produced by the light source to scan in a second direction that is normal to the first direction, an image formation means by which an image constituted by continuous or discontinuous dots is formed on the photosensitive body following scanning in the first and second directions and a speed control means which controls the speed of light beam scanning brought about by the first scanning unit in a manner such that the density of dots successively formed in the second direction is made variable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light beam scanning apparatus whereina light beam from a semiconductor laser or similar light source ismodulated by image information signals and an image is formed byscanning a photosensitive body with the resulting light beam.

2. Discussion of Background

Image resolution is varied in a number of ways in conventional laserprinters incorporating light beam scanning apparatus. With prior artmethods, however, the laser printer's resolution is fixed at a certainvalue on shipment from the factory and it is not possible for thegeneral user to alter or adjust this resolution, as he sees fit, Thishas meant that there has caused the problem that, for example whenvarying degrees of resolution are needed that, complex data signalprocessing is necessary, in order to effect image formation by laserbeams on the basis of data from an information reading apparatus.

SUMMARY OF THE INVENTION

The present invention has been devised in view of the above situationand has as one of its object to provide a light beam scanning apparatusin which the image resolution can be easily altered by the user withoutrequiring a change in hardware.

It is a further object of the invention to provide a light beam scanningapparatus in which the dot density in the light beam's subsidiaryscanning direction, which is an essential aspect of alteration of theresolution, of the apparatus can be changed.

It is yet another object of the invention to provide a light beamscanning apparatus which permits alteration of the dot density in thelight beam's principal scanning direction.

The light beam scanning apparatus for achieving the above objectsconsists of a light source which produces light beams, a first scanningunit which causes the light beams produced by the light source to scanin a first direction with respect to a photosensitive body, a secondscanning unit which causes the light beams produced by the light sourceto scan in a second direction that is normal to the first direction, animage formation means by which an image constituted by either continuousor discontinuous dots is formed on the photosensitive body followingscanning in the first and second directions and a speed control meanswhich controls the speed of light beam scanning brought about by thefirst scanning unit in a manner such that the density of dotssuccessively formed in the second direction is made variable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram showing the relations between the lightbeam scanning apparatus of the invention and external units;

FIG. 2 is schematic cross-section showing an example of use of the lightbeam scanning apparatus of the invention in a laser printer;

FIG. 3 is a schematic perspective view showing the relation between thescanning unit of the laser printer shown in FIG. 2 and a photosensitivebody;

FIG. 4 is a characteristic plot showing the relation between a laserbeam's intensity distribution and image characteristics;

FIG. 5 is a schematic view showing laser beam scanning action in mainand subsidiary scanning directions;

FIG. 6 is a block diagram of a light control unit in the light beamscanning apparatus of the invention;

FIG. 7 is a block diagram of a scanning speed control unit forcontrolling scanning speed in the main scanning direction;

FIG. 8 is a block diagram of a frequency control unit that effectsvariable control of the frequency of information signals;

FIG. 9 is a block diagram showing the relations between the controlunits of FIGS. 6-8 and a host system;

FIG. 10 is a drawing showing the relations between the control units ofFIGS. 6-8 and a changeover unit;

FIG. 11 is a schematic view showing examples of a tone display in whichthe number of dots and the diameter of dots are varied; and

FIG. 12 is a schematic view showing examples of a tone display in whichthe diameters of a plurality of dots are varied.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A description will now be given with reference to an embodiment applyingthe invention that is shown the drawings.

FIG. 1 is a block diagram of a system for recording information on arecording medium by means of a laser beam. Information from a hostsystem (electronic computer or word processor main unit, etc.) thatsupplies information is supplied to a data control section 2, where theinformation supplied from host system 1 is converted todot-correspondence data and stored in a page memory. This dot-image datais sent to a printing control section 100, where a laser beam ismodulated by the input dot-image data, the modulated laser beam iscaused to scan a photosensitive body to form an image constituted bydots, this image is developed and the developed image is transferredonto recording paper.

FIG. 2 shows details of the structure of a laser printer possessing avideo interfare. Printing control section 100 of FIG. 1 is contained ina printer main body 300. In a central portion of printer main body 300,there is a photosensitive drum 301 which can form an image consisting ofdots as a result of being scanned by a laser beam and which is rotatedat high speed by a motor M as shown in FIG. 3. A charge removal lamp 302for effecting removal of charge from photosensitive drum 301 in theinitial state is disposed at an upper portion of the periphery ofphotosensitive drum 301. The charge removal lamp 302 consists of aplurality of red LEDs. There is also a charge removal lamp 303,similarly constituted by a plurality of red LEDs, that is provided at alower right-hand portion of the periphery of photosensitive drum 301 toimprove the efficiency of transfer onto copy paper of toner images thathave been developed on the photosensitive drum 301. At an upperright-hand portion of the periphry of photosensitive drum 301, there isa main charger 304 for uniformly charging the photosensitive drum 301 toa set potential. At a lower portion of the periphery of photosensitivedrum 301, there is a transfer charger 305 for effecting transfer of atoner image developed on photosensitive drum 301 onto copy paper and apeel-off charger 306 for effecting the separation of the copy paper fromphotosensitive drum 301 following the transfer of an image. To the rightof the periphery of photosensitive drum 301, there is provided adeveloping unit 307 for developing an electrostatic latent image thathas been formed as the result of scanning of photosensitive drum 301 bya laser beam. Developing unit 307 comprises a magnet roller 308 whichcauses a toner to adhere to an electrostatic latent image onphotosensitive drum 301. Adjacent to the magnet roller 308, there isprovided a probe 309 which contacts the developer that adhers to themagnet roller 308 and which serves to measure the toner specific densityof the developer. At the left-hand side of the periphery ofphotosensitive drum 301, there is provided a blade 310 forremovingexcessive toner that remains on photosensitive drum 301 after transferof a toner image onto copy paper that has placed in contact withphotosensitive drum 301.

A laser scanning unit 311 is provided in the upper right-hand portion ofmain body 300. Unit 311 is the unit which modulates a laser beam withimage information signals input from the data control section andeffects the scanning of photosensitive drum 301 to form theelectrostatic latent image and it consists of an octahedral polygonmirror 312 by which a laser beam produced by a laser diode constitutinga light source is led onto photosensitive drum 301. A motor 313 iscoupled to the polygon mirror 132 and serves to rotate it at high speed.An f.θ lens is provided in order to make the speed of laser beamscanning on photosensitive drum 301 constant. There are also reflectingmirrors 315 and 316 which are disposed in a manner such that lightexiting from f·θ lens 314 is led onto photosensitive drum 301.

An upper-stage cassette 317 able to hold 500 sheets of copy paper and alower-stage cassette 321 able to hold 250 sheets of copy paper aredetachably installed in a lower right-hand portion of main body 300. Apaper feed roller 318 which contacts paper accommodated in cassette 317and serves to take out this paper is provided in correspondence with theupper-stage cassette 317. Adjacent in the paper feed roller 318, thereis provided a paper run-out detector 319 which detects when paper in theupper-stage cassette 317 has run out. An upper-stage cassette sizedetector 320 for detecting the cassette type, which represents the sizeof accommodated paper, is provided in a position facing the front edgeof upper-stage cassette 317. The lower-stage cassette 321 is similarlyprovided with a paper feed roller 322, paper run-out detector 323 and alower-stage cassette size detector 324. Above the upper-stage cassette317, there is a manual insertion guide 325 to permit copy paper to beinserted by hand. A manually-inserted paper supply switch 326 by whichinserted paper is detected is provided forward of this guide 325. Apaper feed roller 328 for manual insertion is provided forward of switch326, and the actuator 328 of a manual stop switch that detects paperthat has been forwarded by paper feed roller 327 is provided forward ofpaper feed roller 327.

Adjacent to the photosensitive drum 301, there are position-adjustmentrollers 329 for synchronizing the front edge of the paper that has beentaken from the upper-stage cassette 317, the lower-stage cassette 321 orfrom the manual insertion guide 325 with a toner image that has beendeveloped on the photosensitive drum 310. A toner image is transferredfrom photosensitive drum 301 onto the copy paper by the action oftransfer charger 305, then the paper is separated from drum 301 by theaction of the peel-off charger 306. A forwarding belt 330 for forwardingthe separated paper is provided to the lower left of drum 301. Followingon from an end portion of forwarding belt 330, there is the fixing unit331, and a toner image on copy paper that has been forwarded byforwarding belt 330 is fixed by way of heating. The fixing unit 331consists of fixing rollers 332 with a heater lamp 333 inside and athermistor 334 for detecting the surface temperature of the fixingroller 332. After fixing unit 331, in a left-hand portion of theinterior of main body 300, there are paper feed-out rollers 335 and apaper feed-out switch 336 which detects paper that has been fed out. Adelivery tray 397 which receives paper that has been fed out by paperfeed-out rollers 335 is mounted on the exterior of main body 300, on itsleft-hand side.

A cooling fan 337 for cooling the interior of main body 300 is providedin an uppr left-hand portion inside the main body 300, and a highvoltage transformer 338 and power supply unit 339 are provided in abottom portion inside the main body 300. A high voltage transformer 338produces a high voltage that is supplied to chargers 304, 305 and 306and to the developing unit 307. A power supply unit 339 produces DCvoltage for control purposes. A PC board unit 340 for controlling thefunctions of the main body 300's operation is provided inside the mainbody 300 is an upper left-hand portion thereof.

Photosensitive drum 301 is provided with a temperature sensor 342installed adjacent to the drum and serves to detect the drumstemperature. A thermistor with a very small thermal resistance is usedfor this sensor 342.

FIG. 3 is a perspective view showing the outline of parts for formationof an image on photosensitive drum 301 through laser beam scanning. InFIG. 3, laser beams coming from a semiconductor laser 344 are correctedto parallel light by a collimator lens 343 and this parallel lightstrikes one of the eight surfaces of the polygon mirror 312, which isrotated at high speed in the direction of the arrow by scanning motor313. Laser beams that have struck the polygon mirror 312 are passedthrough f. θ Lens 314 and caused to scan a scanning range 348 from leftto right (the principal scanning direction). One portion of the laserbeams in beam scanning range 348 is directed to a beam detector 346 by areflecting mirror 345, whereby laser beams that are being caused toeffect scanning are detected once in each horizontal scan, off onesurface of polygon mirror 312. Laser beams inside the beam scanningrange 348 that do not strike the reflecting mirror 345 are radiated ontophotosensitive drum 301. The portion of photosensitive drum 301 scannedby laser beams is indicated by a two-dot chain line in FIG. 3. Wherereference 304 indicates an electrifying charger and reference 346 thecopy paper. As shown in FIG. 2, in an actual printer, laser beams thathave passed through f·θ lens 314 are not radiated directly ontophotosensitive drum 301 but are led to photosensitive drum 301 by beingreflected by reflecting mirrors 315 and 316. For convenience, FIG. 3does not show these reflecting mirrors 315 and 316 but shows things asif laser beams that have passed through f·θ lens 314 are radiateddirectly onto photosensitive drum 301.

The relation between the intensity distribution of laser beams scanningphotosensitive drum 301 and image characteristics will now be describedwith reference to FIG. 4. In FIG. 4, I is the spatial distribution onphotosensitive drum 301 of scanning exposure energy resulting fromscanning of photosensitive drum 301 by laser beams from a semiconductorlaser in which the light energy distribution takes the form of a normaldistribution, II is the photosensitivity characteristic ofphotosensitive drum 301, III is the reversal development characteristic(including the effects of transfer and fixing) and IV indicates theimage characteristics. Image characteristics IV is obtained bysuperposition of photosensitivity characteristic II and reversaldevelopment characteristic III on spatial distribution I of scanningexposure light.

The scanning exposure characteristic indicated by a full line in thedrawing is a characteristic simulated for a laser output of 5 mW, whilethe scanning exposure characteristic indicated by a dashed line is acharacteristic simulated for a laser output of 8 mW. Comparing the imagecharacteristics corresponding to these scanning exposurecharacteristics, one sees that the larger laser output results in athicker line. (The abscissa of the image characteristic represents thedistance in the subsidiary scanning direction normal to the mainscanning direction).

In other words, if one effects variable control of the laser power oflaser beams, one can vary the thickness of lines and the size of points(dots).

Next, the relation between laser power and pitch in the subsidiaryscanning direction will be considered.

On continuous scanning by a laser beam from left to right as shown inFIG. 5, laser beam intensity I is approximately ##EQU1## where, ηopt:scanning system's total optical coefficient

I_(o) : maximum laser intensity

α,β: Gaussian coefficients

X: subsidiary scanning direction coordinate

Y: principal scanning direction coordinate

The laser beam power Pl at this time is ##EQU2##

Designating principal scanning direction speed as Vs, subsidiaryscanning direction speed as Vp and the scanning pitch is P then

    VS=Vp·l/P                                         (3)

The energy density to which the photosensitive drum is exposed is thesum of the energy produced in the various scans and the energy at thenth scan is ##EQU3## Since, as a practical question, Yo and l aresubstantially greater than the laser spot diameter, the range of Eq. (4)can be replaced by ±∞.

Designating laser power as Pl, the total amount of exposure lightresulting from all scans is ##EQU4## where ##EQU5## The extreme casehere is when the maximum value of the amount of exposure light is X=nPand the minimum amount of exposure light is X=P(n-1/2), this minimumvalue of exposure light being ##EQU6##

What is used in practice in image output apparatus are the ranges αP≦2.0and β≦2.0, and this can be approximated as follows.

    E(p/2)≃Pl·θ.sub.opt /(l·Vp) (8)

Eth is taken to be the exposure energy correponding to the locationwhere the boundary between black and white appears in the image of FIG.4. When E(P/2)=Eth, the minimum laser power Pmin at this time is givenby Eq. (8).

    Pmin=Eth·l·Vp/η.sub.opt              (9)

Use of this definition of Pmin permits Eq. (5) representing the totalamount of exposure of the surface of a uniformly scanned photosensitivebody to be normalized as follows. ##EQU7##

There now follows a discussion of the relation between laser power andprinted characters in different situations. In a situation with a 1,11-dot line in the principal scanning direction (White with normaldevelopment, black lines with reversal development) would be the casewhere n=0 in Eq. (10), the places where ##EQU8## would constitute theboundary between black and white.

In a situation with a 1,2 1-dot line in the principal scanning direction(Black with normal development, white lines with reversal development)

One would simply need to subtract the equivalent of 1 scanning line fromthe total scanned location. The places where ##EQU9## would constitutethe boundary between black and white.

By a similar approach, one can determine the boundary between black andwhite by the following formulas. In a situation with 2,1 1-dot line insubsidiary scanning direction (Black with normal development, whitelines with reversal development)

The width P on a scanning line is ##EQU10## and the width P betweenscanning lines is ##EQU11## where ##EQU12## with a 2,2 1-dot line in thesubsidiary scanning direction (white with normal development, blacklines with reversal development) the width P on a scanning line is##EQU13## and the width P between scanning lines is ##EQU14##

In a situation of a 3,1 1-dot (White with normal development, blacklines with reversal development) ##EQU15## the width P on a scanningline

In a situation of a 3,2 1-dot (Black with normal development, whitelines with reversal development) the width P on a scanning line is##EQU16## and the width P between scanning lines is ##EQU17##

It is seen in Eqs. (11)-(20) above that the optimum pitch P for givingthe clearest image can be changed by changing the laser beam power Pl(since α,√π and Pmin are constants). from Eq. (3) above, the pitch P canbe changed by varying the laser beam's principal scanning speed Vs orthe speed Vp in the subsidiary scanning direction.

Use of the above facts made the following clear:

(1) that dot diameter can be varied by varying the lower power Pl, andchanging the dot diameter results in a change in the thickness of thelines appearing as an image.

(2) If the speed Vp in the subsidiary scanning direction (rotationalspeed of photosensitive drum 301) is made constant, the pitch P can bechanged by changing the speed Vs in the principal scanning direction.Hence, one can change pitch P by varying the speed of rotation ofpolygon mirror 313. That is, the dot density in the direction ofsubsidiary scanning can varied, and it is greater as the rotationalspeed of polygon mirror 313 is higher and smaller as this rotationalspeed is lower. In contrast, the dot density in the principal scanningdirection becomes smaller as the rotational speed of polygon mirror 313is made faster and larger as the rotational speed of polygon mirror 313is made slower. However, the dot density in the principal scanningdirection can be adjusted independently of the rotational speed byvarying the image signal frequency. More specifically, increasing theimage signal frequency results in a greater dot density and making thefrequency lower results in a smaller dot density.

From (1) and (2) above, it is possible to vary the image resolution.When the resolution is increased, i.e., when the dot density isincreased, it is preferable to make the dot diameter correspondinglysmaller. Conversely, when the resolution is reduced, i.e., when the dotdensity is made smaller, the dot diameter must be made larger. Variationof resolution in the principal scanning direction can be effected bychanging the dot diameter and the image signal frequency, and variationof resolution in the subsidiary scanning direction can be effected byvarying the dot diameter and the speed Vs in the principal scanningdirection. Variation of the resolution in the principal and subsidiaryscanning directions can be dealt with by varying the dot diameter, theimage signal frequency and the speed Vs in the principal scanningdirection.

As laser power control unit 209 constituting a light source lightingcontrol unit for effecting variable control of laser beam dot diameterwill now be described with reference to FIG. 6. Laser power control unit209 effects the variable control of laser power through the current thatflows in a semiconductor laser 200. There is provided a light detectionsection 201 which monitors the strength of the output beam ofsemiconductor laser 200 by means of a photodiode, etc. and outputs avoltage that is in proportion to this output beam strength. A lightquantity setting section 205 permits the setting of a voltage that is atleast sufficient to give solid black when a dot diameter control signaldescribed below corresponds to a maximum diameter. A divided voltage isoutputed on the basis of dot diameter control signals, taking thisminimum sufficient voltage as a reference. A comparison andamplification section 203 compares the voltage from light detectionsection 201 and the voltage from light quantity setting section 205 andoutputs an analog level voltage corresponding to the voltage from lightquantity setting section 205. This output serves to stabilize the amountof light of semiconductor laser 200 at a set value. The output voltageof comparison and amplification section 203 is input to a sampling andhold section 204, where it is stored in a holding capacitor (not shown)when sampling and hold section 204 receives the input of a sampling holdsignal. When the sampling hold signal is turned off (hold), the storedvoltage is continuously output at the output of the sampling and holdsection. A current amplification section 206 amplifies current inproportion to the output voltage of sampling and hold section 204. Alaser drive section 207 drives the semiconductor laser 200 on the basisof data from an image data latch section 208 and in accordance with thelevel of analog level signals output by current amplification section206. Thus, light quantity setting section 205 sets different voltages bydividing a reference voltage in response to dot diameter controlsignals, making it possible to vary the current flowing in semiconductorlaser 200. The laser power of the laser beam emitted by semiconductorlaser 200 is therefore varied, hence making it possible to vary thethickness of lines that appear as an image. As it is thus possible tochange the thickness of lines by changing the actual dot diameteritself, there is no need to change the capacity of the image memory ashas been the case in the past.

There now follows a description of a means for changing the speed Vs inthe principal scanning direction in order to change the dot density inthe subsidiary scanning direction.

The speed Vs in the principal scanning direction can be dealt with byvarying the rotational speed of polygon mirror 313. FIG. 7 is a blockdiagram of a scanning speed control unit 219, which is a unit for drivecontrol of scanning motor 312 that drives polygon mirror 313. Referringto FIG. 7, in this embodiment, PLL (Phase-Locked Loop) control ofscanning motor 312 is effected by an IC 214. A reference frequencygeneration circuit 210 connected to PLL control IC 214 consists of aquartz oscillator 211 and a programmable frequency dividing circuit 212by which the oscillation output of quartz oscillator 212 is divided at aset frequency division ratio. The frequency division ratio for frequencydividing circuit 212 is set by the scanning speed control signals. Theinput of the laser beams horizontal synchronization signals are also fedto the programmable frequency dividing circuit 212 at a set inputterminal not shown, so as to effect horizontal synchronization. Awaveshaping circuit 213 effects waveshaping, and then the output, offrequency of an FG (Frequency Generator) of the scanning motor 312. PLLcontrol IC 214 consists of PLL control circuit 215 and a speed controlcircuit 216. PLL control circuit 215 outputs a voltage that isproportional to the phase difference between the abovenoted referencefrequency and the frequency of the FG pulses, while the speed controlcircuit 216 outputs a voltage that is proportional to the frequencydifference of the reference frequency and the FG pulses. The outputsboth of PLL control circuit 215 and of the speed control circuit 216 arefixed at a high level when the FG pulses are below the lock range andare fixed at a low level when the FG pulses are above the lock range.When the FG pulses are within the lock range, the output from PLLcontrol circuit 215 is proportional to the phase difference and theoutput from speed control circuit 216 is proportional to the frequencydifference and these outputs are added in a set proportion. Adifferential amplification circuit (not shown) controls and effects thepulse width modulation of the potential difference. A motor drivecircuit 217 imposes a DC voltage corresponding to this pulse widthmodulation on the coil of scanning motor 312, thereby effecting drivecontrol of scanning motor 312.

Thus, the rotational speed of scanning motor 312 can be changed throughsetting of the frequency division ratio in programmable frequencydividing circuit 212 by scanning speed control signals and changing ofthe reference frequency. As a result, the laser beam's principalscanning speed Vs is changed and, as described above, and the pitch P asshown in FIG. 5 is changed, thereby changing the dot density in thesubsidiary scanning direction, and it is thus made possible to effectvariation of resolution in the subsidiary scanning direction also. Thevariation of the dot density in the subsidiary scanning direction alsomakes it possible to vary the magnification of the image (themagnification relating to the subsidiary scanning direction) withcontrol that is much easier than would be with conventional enlargementand reduction processing.

There now follows a description of a means for changing the image signalfrequency in order to change the dot density in the principal scanningdirection. FIG. 8 is a block diagram of an image signal frequencycontroller 220 which consists of a quartz oscillator 221 and aprogrammable frequency dividing circuit 222. The division ratio forprogrammable frequency dividing circuit 222 is set by frequency controlsignals. Horizontal synchronization may be effected by inputtinghorizontal synchronization signals as set input for the programmablefrequency dividing circuit 222.

Thus, it is possible to set the division ratio in programmable frequencydividing circuit 222 to set values by means of frequency control signalsand to vary the image signal frequency. The dot density in the laserbeam's principal scanning direction can be made large by making theimage signal frequency high and be made small by making the image signalfrequency low, and it is therefore made possible to effect variation ofresolution in the principal scanning direction. The variation of the dotdensity in the principal scanning direction also makes it possible tovary the magnification of the image (the magnification relating to theprincipal scanning direction) with control that is much easier thanwould be possible with conventional enlargement and reductionprocessing.

As described above, the laser beam dot diameter can be varied throughcontrol by laser power control unit 209. The dot density in the laserbeam's subsidiary scanning direction can be varied by control ofscanning speed control unit 219. The dot density in the laser beam'sprincipal scanning direction can be varied by control of image signalfrequency control unit 220. Possible systems for controlling laser powercontrol unit 209, scanning speed control unit 219 and image frequencycontrol unit 220 are the two systems shown in FIG. 9 and in FIG. 10.FIG. 9 shows a system in which units 209, 219 and 220 are controlled byhost system 1, i.e., it is a system in which dot diameter controlsignals, scanning speed control signals and frequency control signalsare output by host system 1.

FIG. 10 is a system in which variable control of dot diameter, scanningspeed and image signal frequency is effected by means of a changeovermeans 230 constituted by dipswitches, etc. incorporated in the laserprinter. Changeover control by changeover means 230 is advantageous whenthe laser printer is not connected to host system 1 but itself possessesan original document reading apparatus.

If scanning speed control and image signal frequency control areeffected together with dot diameter control when the resolution isvaried, it is possible to vary the dot density in both the principal andsubsidiary scanning directions and also to effect variation of the dotdiameter in correspondence to this. This constitutes the most preferablemethod, but if required it is possible to vary the resolution byeffecting independent adjustment of only the dot diameter in theprincipal scanning direction or only the dot diameter in the subsidiaryscanning direction. This control contributes to variation of resolutionwhen equal magnification images are outputed and it is also of advantagethat variations in the dot density in the principal and subsidiaryscanning directions make it possible to change the transverse orlenghways magnification of images.

Toning achieved by means of the abovedescribed dot diameter control willnow be described with reference to FIGS. 11 and 12. FIG. 11 shows anexample of a multi-value area tone display in which the dot diameter ofeach dot inside a picture element is changed. This control can beeffected by laser power control unit 209. Stepwise changes in the sizeof the dot diameter in this way make finer toning possible and makes itpossible to output attractive, natural images even in the case ofpictures, patterns or photographs, etc.

FIG. 12 is an example of a tone display in which dot diameter control iseffected in a conventional two-value area tone display. A "two-valuearea tone display" is a display in which toning is effected by varyingthe number of dots in a picture element, as shown in the upper row inFIG. 12. Adding dot diameter control to this increases the degree oftoning and, like the case shown in FIG. 11, makes it possible to formattractive, natural images even in the case of pictures, patterns andphotographs, etc. The tone display shown in FIG. 12 is achieved byvarying the number of dots and varying the diameter of each dot inside apicture element. This control also can be effected by laser powercontrol unit 209.

The invention is not limited to the abovedescribed embodiment but it ispossible to practise a variety of modifications within the scope of theinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. Light beam scanning apparatus in which lightbeams are modulated with image information signals and the modulatedlight beams are radiated onto a photosensitive body comprising:a lightsource for producing light beams; a first scanning means comprising arotating mirror for causing the light beams produced by said lightsource to effect scanning at a first speed and in a first direction withrespect to a photosensitive body; a second scanning means for causingthe light beams produced by said light source to effect scanning at asecond speed and in a second direction which is perpendicular to saidfirst direction; an image formation means for forming an imageconstituted by continuous or discontinuous dots on said photosensitivebody after scanning in said first and second directions; and a speedcontrol means for controlling the speed of the rotation of said rotatingmirror and the light beam scanning in said first direction caused bysaid first scanning means in a manner such that the density of dotssuccessively formed in said second direction is variable.
 2. Light beamscanning apparatus as in claim 1, wherein said speed control meanscomprises:a phase-locked loop circuit connected to said motor in amanner such as to make the reference frequency of said motor variableand for control of the speed of said motor.
 3. Light beam scanningapparatus as in claim 2, wherein said speed control means comprises:areference frequency circuit connected to said phase-locked loop whichcomprises a quartz oscillator and a programmable frequency dividingcircuit which effects frequency division of the oscillation output ofsaid quartz oscillator, said motor's reference frequency being changedby changing of the frequency division ratio of said programmablefrequency division circuit.
 4. Light beam scanning apparatus as in claim1, further comprising:a frequency control means for variable controllingthe frequency of image information signals modulating said light beamsis effected in a manner such that the density of dots successivelyformed in said first direction becomes variable wherein said frequencycontrol means comprises: a quartz oscillator; and a programmablefrequency dividing circuit which effects frequency division of theoscillation output of said quartz oscillator and whose frequencydivision ration is variably controlled.
 5. Light beam scanning apparatusin which light beams are modulated with image information signals andthe modulated light beams are radiated onto a photosensitive bodycomprising:a light source for producing light beams in which the lightenergy distribution displays a normal distribution; a first scanningmeans comprising a rotating mirror for causing the light beams producedby said light source to effect scanning at a first speed and in a firstdirection with respect to a photosensitive body; a second scanning meansfor causing the light beams produced by said light source to effectscanning at a second speed and in a second direction which isperpendicular to said first direction; an image formation means forforming developed image constituted by continuous or discontinuous dotson said photosensitive body after scanning in said first and seconddirections; a light source lighting control means connected to saidlight source for lighting said light source and for changing thediameter of the light beam dot should be developed on saidphotosensitive body by changing said normal distribution; a speedcontrol means for controlling the speed of the rotation of said rotatingmirror and the light beam scanning in said first direction caused bysaid first scanning means in a manner such that the density of dotssuccessively formed in said second direction is variable; and afrequency control means for variable controlling the frequency of imageinformation signals modulating said light beams is effected in a mannersuch that the density of dots successively formed in said firstdirection becomes variable.